The present disclosure relates to image forming apparatus that performs the image forming by overlaying at least two of image in each color formed for every color.
In recent years, the image forming apparatus corresponding to full color is widely diffused. In such full color image forming apparatus, the full color image is formed by overlaying individual color images (toner images) formed for every color, C (cyan), M (magenta), and Y (yellow). One of the image forming methods is a tandem electrographic in which each color toner image in C (cyan), M (magenta), Y (yellow) and K (black) is formed by respective image forming units. In the tandem electrographic, the image forming units for CMYK colors are disposed along a traveling direction of the transfer belt. The timing of forming the toner image by each image forming unit is controlled so as to transfer the toner images formed by the image forming units to a same position of the transfer belt.
In such image forming apparatus for forming one image by overlaying the individual images formed for every color, when the misalignment of the overlaid color images (namely, the color misregistration) occurs, the quality of image thus formed decrease remarkably. The occurrence of the color misregistration is caused by the change of the position relation between the optical units for guiding the light beam to form the electric latent image on the photosensitive drum in each image forming unit depending on the ambient environment like the temperature. Therefore, such kind of image forming apparatus is configured so as to form appropriately on the transfer belt a pattern for detecting the color misregistration, and correct the color misregistration according the detection result.
However, the color misregistration correction requires forming the color misregistration detection pattern for each color, and it takes a long time for executing the color misregistration correction. Accordingly, in case of increasing the frequency of the color misregistration correction, the deterioration of the picture quality can be suppressed but the performance of the image forming apparatus is reduced extremely. In case of extending the interval of the execution of the color misregistration correction, it is hard to suppress the deterioration of the picture quality caused by the color misregistration. Therefore, various kinds of technique for the color misregistration correction have been proposed.
There is an image forming apparatus having a LSU (Laser Scanning Unit) wherein only one polygon mirror deflects the light beam emitted from the light source corresponding to each color, the color misregistration correction is carried out for each group of colors sharing an optical element by means of a representative color of the group.
In the other image forming apparatus, the color misregistration correction is carried out by estimating the misalignment reading amount of the reference pattern in color, the color for which the reference pattern is not formed, according to the misalignment reading amount of the reference pattern formed by some units of imaging units and the misalignment reading amount of the reference patterns in every colors that have been stored in the misalignment reading storage unit.
Moreover, in the image forming apparatus including a three-color-integrated type transfer member, which is capable for switching the monochrome printing and the color printing, the color registration correction is carried out by using the registration marks formed in black and one color of the three color-integrated type transfer member.
In addition, in the other image forming apparatus, a rough adjustment of the color misregistration is carried out by using registration marks formed in black and one of colors to be used to the color image formation, and after that, a fine adjustment of the color misregistration for every color is carried out on the basis of the reference color that is the color used to the rough adjustment.
Regarding the scanning optical unit (LSU) to be implemented to the tandem type of image forming apparatus, there are various structures. For instance, there is a structure wherein the scanning optical unit including one light source, one polygon mirror and one light-emitting optical system is arranged for each imaging unit. There is another structure wherein the scanning optical unit including four light sources, one polygon mirror and two light-emitting optical system (f-θ lens) is arranged for all the imaging units. The latter structure has an advantage that the number of components can be reduced in comparison to the former structure. But, the optical system becomes complicated since the two light beams shares the same light emitting optical system, and it is not easy to adjust the optical system.
There is still another structure wherein one scanning optical unit including two light sources, one polygon mirror, and two light-emitting optical systems disposed facing each other across the polygon mirror is disposed for two imaging units. Specifically, two scanning optical units are placed for four imaging units. In such structure, the number of polygon mirrors can be reduced in comparison to the structure arranging the scanning optical unit for each imaging unit, and the optical system does not become complicated excessively like the structure arranging one scanning optical unit for all the imaging units.
In the image forming apparatus applying those scanning optical units, the color misregistration correction is indispensable. In the image forming apparatus in the present disclosure, the relation between the picture quality and the performance of the image forming apparatus is a trade-off relation as described above, and there is a problem to be solved.
However, it is difficult to apply the above-mentioned color registration correction using only the representative color of the color group to the configuration in the present disclosure wherein the light-emitting system is not shared. The method for estimating the misalignment reading amount of the reference pattern in color for which the reference patterns is not formed is deemed to be applicable to the image forming apparatus in the present disclosure, but the apparatus is required to have the misalignment reading storage unit for storing the misalignment reading amount for the reference patterns in all colors. The technology using a three-color-integrated type transfer member cannot be applied to the present disclosure if the color transfer member is not the three-color-integrated type. Moreover, in case of the technology of the color misregistration correction by executing the fine adjustment for every color after the rough adjustment using the registration mark in one color, the reference patterns for all colors are formed at the fine adjustment, so that the effect of suppressing the performance reduction is relatively small. Therefore, in the image forming apparatus in the present disclosure, the above-mentioned technologies do not function adequately as the technology for suppressing the deterioration of the picture quality and the reduction of performance.